US6567480B1 - Method and apparatus for sampling timing adjustment and frequency offset compensation - Google Patents
Method and apparatus for sampling timing adjustment and frequency offset compensation Download PDFInfo
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- US6567480B1 US6567480B1 US09/371,529 US37152999A US6567480B1 US 6567480 B1 US6567480 B1 US 6567480B1 US 37152999 A US37152999 A US 37152999A US 6567480 B1 US6567480 B1 US 6567480B1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/0054—Detection of the synchronisation error by features other than the received signal transition
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
- H04L27/22—Demodulator circuits; Receiver circuits
- H04L27/227—Demodulator circuits; Receiver circuits using coherent demodulation
- H04L27/2275—Demodulator circuits; Receiver circuits using coherent demodulation wherein the carrier recovery circuit uses the received modulated signals
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
- H04L2027/003—Correction of carrier offset at baseband only
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
- H04L2027/0036—Correction of carrier offset using a recovered symbol clock
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0044—Control loops for carrier regulation
- H04L2027/0046—Open loops
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0044—Control loops for carrier regulation
- H04L2027/0063—Elements of loops
- H04L2027/0065—Frequency error detectors
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L7/00—Arrangements for synchronising receiver with transmitter
- H04L7/0054—Detection of the synchronisation error by features other than the received signal transition
- H04L7/0066—Detection of the synchronisation error by features other than the received signal transition detection of error based on transmission code rule
Definitions
- the present invention relates generally to a method and apparatus for communication, and more particularly, to a method for sample timing adjustment and frequency offset estimation and compensation, and to a radio system having sample timing adjustment means and frequency offset estimation and compensation means.
- the present invention relates generally to signal recovery in communication systems and is particularly applicable to, and is described below in the context of, a digital cellular communication system such as the North American TDMA (Time Division Multiple Access) cellular communication system compatible with EIA/TIA documents IS-54 (Revs. A and B) and IS-136.
- TDMA Time Division Multiple Access
- EIA/TIA documents IS-54 Revs. A and B
- IS-136 IS-136
- a conventional wireless radio system used for telephony consists of three basic elements—namely, mobile units, cell sites, and a Mobile Switching Center (“MSC”).
- MSC Mobile Switching Center
- a basic cellular system a geographic service area, such as a city, is subdivided into a plurality of smaller radio coverage areas, or “cells”.
- a mobile unit communicates by radio frequency (RF) signals to the cell site within its radio coverage area.
- RF radio frequency
- the cell site's base station converts these radio signals for transfer to the MSC via wire (landline) or wireless (microwave) communication links.
- the MSC routes the call to another mobile unit in the system or the appropriate landline facility.
- PSTN public switched telephone network
- a mobile unit contains a radio transceiver, a user interface portion, and an antenna assembly, in one physical package.
- the radio transceiver converts audio to a radio frequency signal for transmission to a cell site and converts received RF signals into audio.
- the user interface portion includes the display and keypad which allow the subscriber to communicate commands to the transceiver.
- the antenna assembly couples RF energy between the electronics within the mobile unit and the “channel”, which is the outside air, for transmission and reception.
- Each mobile unit has a Mobile Identification Number (“MIN”) stored in an internal memory referred to as a Number Assignment Module (NAM).
- MIN Mobile Identification Number
- NAM Number Assignment Module
- a cell site links the mobile unit and the cellular system switching center, and contains a base station, transmission tower, and antenna assembly.
- the base station converts the radio signals to electrical signals for transfer to and from a switching center.
- Digital cellular technology in which information consisting of voice and data is digitally encoded onto an RF carrier signal, or systems which are compatible with digital and analog cellular communication standards are currently more popular than analog systems.
- TDMA Time Division Multiple Access
- FDMA Frequency Division Multiple Access
- CDMA Code Division Multiple Access
- IS-54 (revs. A and B), and the more current Interim Standard for time division multiplexed wireless telephony, IS-136, use Time Division Multiple Access (TDMA) digital technology to effectively increase the limited bandwidth available for cellular communications.
- TDMA Time Division Multiple Access
- TDMA Time Division Multiplexed basis
- sync sync
- Communication from a cell site to mobile units is performed on a time division multiplexed basis whereby each cellular channel is used within each cell to facilitate simultaneous communication with 3 to 6 mobile units.
- 3 to 6 users share a common 30 kHz channel in TDMA operation.
- Each user transmits data in an assigned time slot that is part of a larger frame.
- the sync word is used to facilitate timing recovery, i.e., to determine an optimum time for sampling the received signal for further processing to recover the communicated information. It is well known that timing recovery and the necessary processing of the samples are made more difficult by a low signal-to-noise ratio (SNR) and that a low SNR can often be present in cellular communication systems.
- SNR signal-to-noise ratio
- the communicating units In order to receive a transmitted digital signal, the communicating units must determine the beginning and end of signals intended for them, known as frame/slot synchronization.
- the complexity and accuracy of frame/slot synchronization depend upon the number of points at which the signal is sampled and the ability of the system to compensate for signal distortion. Increasing the number of samples per transmitted symbol results in an increase in the accuracy of the receiver at the expense of a higher complexity.
- Another crucial function required in TDMA communication is the need to determine the optimum time within a symbol interval to sample the signal and determine the relative phase angle of the symbol. Once the optimum point for symbol timing is determined, all the symbols within a burst can be demodulated using carrier recovery circuitry and the burst decoded and converted into an analog speech signal or other data.
- a received signal is oversampled at N times the symbol rate, wherein N is an integer.
- N is an integer.
- N sets of samples of a received signal are stored in a decision block, and various known techniques for determining the optimum set of samples may be applied to extract the information contained in the received signal.
- timing tracking or sample timing adjustment serves to avoid cumulative errors of the sampling times during the information sequence, which, if not corrected, can detract from the recovery of the communicated information. Determination of an optimum sampling timing using the sync word does not compensate for signal distortion occurring in a portion of a signal burst separate from the sync word.
- sampling timing adjustment method which is capable of determining optimum sampling timing in a high-loss environment, such as a cellular communication channel, which is sufficiently rapid to effect frequent, high-speed sampling timing adjustments without knowledge of an information sequence in multiplexed or burst communications, such as TDMA cellular communications, and which has minimal processing overhead and hardware requirements.
- phase modulation of a carrier is used to encode information in TDMA cellular systems, unwanted phase-shifts in a modulated signal may render a signal undetectable.
- sampling timing adjustment may not adequately compensate for unwanted phase shift.
- techniques for reducing or eliminating unwanted phase shifts are required in a TDMA cellular system.
- phase rotation In time division multiplexed digital communication systems such as the North American TDMA cellular telephone system, information is typically transmitted as symbols encoded in the phase of the transmitted signal with respect to its carrier.
- the local oscillator frequency used to demodulate the received signal must either be identical to the carrier frequency of the received signal or frequency compensation must be performed on the downconverted signal. Absent compensation for a frequency difference between the carrier of a modulated signal and the local oscillator used to extract the modulated information, the apparent phase relationship “rotates” undesirably.
- Typical methods for the elimination of phase rotation include matching the local oscillator and carrier frequencies, adjusting the frequency of the downconverted signal by an appropriate value, and reconstructing a clock signal used for sampling a downconverted signal based upon the frequency of the carrier.
- synchronous detector the received signal is typically mixed (i.e., multiplied) with a “local oscillator” signal having a frequency that is matched to the carrier frequency of the received signal.
- the local oscillator frequency is “locked” to the carrier frequency of the received signal to eliminate frequency offset therebetween.
- quasi-synchronous detection a detector that is not locked to the carrier frequency of the received signal is used. Frequency offset is corrected by adjustment of the frequency of the downconverted intermediate frequency (IF) or baseband signal by a frequency offset multiplier having a value based upon a detected or estimated frequency offset.
- DQPSK differential quadrature phase-shift keying
- the encoded information is contained in the difference in phase between a given symbol and the previous symbol, rather than in the absolute phase of the modulated symbol.
- a frequency offset between the local oscillator of the receiver and the carrier frequency of the transmitted signal does not present a significant problem in a system employing DQPSK as long as the symbol frequency is much larger than the frequency offset.
- the cellular channel is not ideal, however, and is subject to various types of distortion such as delay spread due to multipath fading, the Doppler effect, flat and frequency-selective fading, additive noise, and the like
- a process such as adaptive equalization, which involves the adaptive characterization of channel distortion, is needed in order to extract symbols accurately from the time-dispersive channel.
- cellular systems typically utilize adaptive equalization techniques which predict the channel response based upon the transmission of known data (e.g., a so-called training sequence)
- known data e.g., a so-called training sequence
- Such processes are themselves sensitive to significant distortion, including frequency offsets, which may cause the channel to vary beyond the rate at which the adaptive processes can adapt. Even for DQPSK systems, therefore, accurate sample timing adjustment and frequency offset compensation is necessary.
- sampling timing adjustment and frequency offset estimation methods utilize the sync word or other known information sequence to effect compensation for signal distortion.
- Such errors may result in an increased bit error rate when optimum sampling timing and frequency offset estimation is determined based solely upon the known information sequence.
- phase correction techniques may not meet the demand for the reduced size and cost requirements of a particular system.
- a simplified correction technique for enhancing, supplementing or replacing known frequency tracking and sampling timing adjustment means is needed.
- a conventional technique used for phase tracking employs a phase-locked-loop (“PLL”).
- PLL phase-locked-loop
- a PLL circuit is typically formed as a phase detector fed by input and feedback signals, a loop filter and a voltage controlled oscillator for producing a sine wave (i.e., the feedback signal).
- the phase of the received signal, or a frequency-translated version thereof e.g., an intermediate frequency (IF) signal
- IF intermediate frequency
- PLL systems tend to require a fair amount of time to achieve phase lock, and the result in a cellular telephone system can be unacceptable.
- Frequency errors may arise, for example, from the transmitter/receiver clock not being perfectly locked due to inaccuracies or drift in the crystal oscillator, as well as from large frequency shifts due to the Doppler effect, such as those occurring from vehicles moving at high speeds in open spaces.
- Many cellular systems allow only a small amount of time for achieving initial signal acquisition and require a minimum tracking error after initial acquisition.
- typical AFC or PLL circuits are not generally able to lock on or track the received signal with a reasonable degree of accuracy.
- the present invention is based upon the recognition that certain modulation techniques exhibit statistical characteristics that are detectable even in the presence of severe channel-induced distortion.
- the time and processing requirements needed to perform sampling timing adjustment can be substantially reduced by determining an optimum sampling timing based upon a minimum deviation from expected differential phase angles between successive pairs of received symbols in an ideal channel.
- frequency offset can also be estimated based upon the same differential phase values calculated for sampling timing adjustment purposes, thereby reducing the necessary hardware and processing requirements for performing sampling timing adjustment and frequency offset estimation.
- sampling timing adjustment is performed in a TDMA cellular system by oversampling a received signal to produce a plurality of sets of samples of the signal, determining for each set of samples the differential phase angle between successive symbols by multiplying pairs of successively received symbols to produce a vector having an angle representing the phase angle therebetween, and determining which set of samples has differential phase angles closest to the expected values under ideal conditions.
- a deviation in differential phase angle is detected from a selected set of samples.
- the expected differential phase angle between successive samples in ⁇ /4-shifted DQPSK, in an ideal channel is either ⁇ 45° or ⁇ 135°.
- a deviation from the expected phase angles is observed, and this deviation is indicative of “phase rotation” due to frequency offset between the local reference and the carrier.
- This phase rotation can then be used to adjust the local oscillator frequency. Since this approach eliminates the need to average over long periods, the filtering out of data-dependent effects, and the use of known data sequences, the time required to achieve adequate frequency-offset compensation is shorter in many important environments than it is for conventional systems.
- FIG. 1 is a diagram of a cellular communication system in which the present invention may be implemented
- FIG. 2 is a block diagram of a receiver in accordance with a first preferred embodiment of the present invention
- FIG. 3 is a block diagram of a sampling timing adjustment circuit in accordance with a preferred embodiment of the present invention.
- FIG. 4 is a flowchart of a method for sampling timing adjustment in accordance with the present invention.
- the present invention is described hereinafter as improvements in sampling timing adjustment and frequency offset estimation for a TDMA cellular communication system, it will be readily appreciated by those skilled in the art that the present invention is not limited to TDMA.
- Other types of communication systems e.g., analog, CDMA, FDMA, GSM, trunked or other landline systems, satellite communication systems, data networks, and the like
- other types of modulation techniques may also be adapted and/or designed to use the principles described herein.
- the description of the present invention as an improvement for TDMA cellular communication is not intended to impart any limitations on the application of the principles described herein to any particular type of communication scheme.
- the sampling timing adjustment and frequency offset estimation techniques of the present invention may be used in connection with any type of communication system using a modulation technique in which a deviation from an ideal values is detectable without the need for a priori knowledge of a received information sequence.
- FIG. 1 a diagram of a conventional TDMA digital telephony system 5 in which the present invention may be practiced is shown.
- the system 5 includes a number of fixed base units 12 , 14 , 16 , 18 , 20 , 22 and 24 and a number of mobile transceivers 26 , 28 , 30 , 32 , 34 , as well as mobile units located in buildings 36 and 38 .
- each mobile transceiver can access a fixed base unit through a separately assigned channel to carry duplex communication on a time division multiplexed (TDM) basis.
- TDM time division multiplexed
- the average power utilized by the mobile transceivers is on the order of 5 to 10 milliwatts or less to provide a range of several hundred to a thousand feet between a base station and its mobile transceivers.
- several base stations are used with individual calls being successively handed off from base station to base station as their corresponding callers carry their mobile units from the service area (i.e., cell) associated with one base station to that of an adjacent base station.
- An appropriate switch (not shown) which is located within a local central end office is suitably programmed to operate in a similar manner as does a mobile telephone switching office in order to controllably handoff calls from one base station to another as the callers transit corresponding local service areas associated therewith.
- the base stations are connected to the switch located within the central office 10 to provide access to the public switched telephone network.
- This connection can typically occur in one of two ways, either through copper digital lines 40 , 42 as in base stations 12 and 16 , or via intermediary copper digital line 42 to remote electronics 44 as in the case of base station 14 .
- the remote electronics 44 contain fixed distribution and concentration facilities for multiplexing traffic, in addition to that provided by the base station 14 , onto fiber feeder 46 which, in turn, feeds central office 10 .
- the switch located within the central office is connected, through a trunk 7 , to the public switched telephone network.
- each base station transmits time division multiplexed bit streams on a predefined carrier frequency using ⁇ /4-shifted DQPSK modulation, with an inter-carrier spacing of 150-300 kHz and within a given operating frequency band.
- Each base station continuously transmits on a time division basis and the mobile transceivers transmit in bursts to their associated base station.
- Two different carrier frequencies are used to carry communication between each base station and a mobile transceiver; for instance, one frequency f 1 is used by base station 12 to carry communication from that base station to each of its mobile transceivers, and another frequency f 2 is used by base station 12 to carry communication from each of the mobile transceivers to this base station.
- adjacent base stations use different pairs of frequencies, these carrier frequencies are also reused for base stations that are spaced sufficiently far apart from each other to conserve spectrum. The spacing is appropriately set to ensure that co-channel interference that might occur at any port will remain at an acceptably low level.
- FIG. 2 is a block diagram of a digital demodulator 100 incorporating sampling timing adjustment and frequency offset compensation means according to the present invention.
- the demodulator 100 is preferably incorporated in both the base stations and the mobile transceivers in a TDMA portable radio communications system. As noted above, both the base stations and the mobile units transmit symbol bursts, the base stations using TDM for transmission of symbol bursts to either three or six users. For purposes of discussion, it will be assumed that the demodulator 100 shown in FIG. 2 represents a demodulator in a mobile transceiver unit.
- An antenna 102 receives each radio transmitted burst having a carrier frequency in the range of 0.5 to 5 GHz.
- An analog receiver 104 amplifies, filters and downconverts the RF signal to a baseband signal.
- a complex sampler 106 samples the IF signal by sampling at four times the symbol rate (or any other desired multiple thereof), The resultant output consists of in-phase (I) and quadrature phase (Q) signals for each symbol, thus resulting a four distinct sets of samples for an information sequence.
- a ROM may be used in subsequent stages of the demodulator as a look-up table of arctangents to determine the phase of each sample from the I and Q signals.
- Subsequent circuitry, including equalization and audio processing circuitry 107 processes the signals to reproduce transmitted voice and information data.
- the analog receiver 104 is a DQPSK receiver in which the input signal from an antenna 102 is applied to a power splitter 108 directly, as shown, or after translation to an intermediate frequency.
- a local oscillator 110 applies its output directly to a first mixer 112 and through a 90° phase shifter 114 to a second mixer 116 .
- Low pass filters 118 and 120 pass only the difference frequencies in the resultant outputs of the mixers 112 , 116 and thereby produce baseband signals representing the in-phase (I) and quadrature phase (Q) components, respectively, of the received signal.
- the complex-valued output of the combination of the baseband I and Q components is applied to the complex value sampler 106 which, in practice, is generally a pair of Analog-to-Digital (A/D) converters which are symbolically illustrated in FIG. 2 as a single sampler (A/D converter) 106 .
- the output of the sampler 106 is supplied to a sampling timing adjustment circuit 121 which operates in accordance with the principles discussed below.
- the sampling timing adjustment circuit 121 is preferably implemented in whole or part by a digital signal processor (DSP) or microprocessor and associated control program and storage device according to the principles described herein.
- DSP digital signal processor
- the sampling timing adjustment circuit 121 is used to select an optimum sampling timing from the four individual sets of samples of received data output by the A/D converter 106 .
- additional means may be used for determining an initial optimum sampling timing based upon a known data sequence, such as a sync word in a TDMA system, in which case the sampling timing adjustment circuit 121 may be used to periodically adjust the initial sampling timing determined by such additional means.
- the sampling timing adjustment circuit 121 may be used to obtain an initial optimum sampling timing based a known (or unknown) information sequence, and to adjust the sampling timing at selected intervals during a communication session. Any known method for obtaining an initial optimum sampling timing may be used in accordance with the present invention.
- a first output of the sampling timing adjustment circuit 121 is supplied to a frequency offset estimation and compensation circuit 122 which may be any known circuit for estimating frequency offset.
- the structure of the sampling timing adjustment circuit 121 will now be described with reference to FIG. 3, which is a block diagram of the sampling timing adjustment circuit 121 of FIG. 2 .
- the output r(i) of the sampler 106 which comprises samples of the oversampled received signal, is supplied to a differential phase calculating circuit 136 .
- the differential phase calculating circuit 136 includes an N-symbol delay circuit 138 for delaying by N symbols the sample r(i) input thereto. In the presently described embodiment, N is equal to 1.
- a complex conjugation circuit 140 determines the complex conjugate of r(i) (i.e., r*(i)), A complex multiplier 142 multiplies the complex conjugate r*(i) of the delayed sample r (i) by the immediately succeeding sample r(i+1) output from the sampler 106 .
- the output of the complex multiplier 142 , z(i) represents the differential phase angle between the succeeding symbols r(i) and r(i+1), which, in the case of ⁇ /4-shifted DQPSK, is either ⁇ 45° 0 or ⁇ 135° in an ideal channel.
- a plurality of pairs of successively received symbols r(i) and r(i+1) are processed in the foregoing manner.
- a separate set of differential phase angle values are produced for each set of pairs of symbols at each sampling period.
- the differential phase angle values of each set of samples of the oversampled burst are stored in a RAM 144 .
- each set of samples is simultaneously processed by the sampling timing adjustment circuit 121 of the present invention.
- the present invention processes each burst (or any desired plurality of samples) to select the particular sampling timing, among the four samples per symbol stored in the RAM 144 , that is most likely to be closest to an optimum sampling instant which has the smallest timing error.
- the optimum sampling timing set determining circuit 146 compares differential phase values for each of the sets of samples obtained at each of the four sampling times with ideal values over the entire burst to select the optimum sampling timing
- the present invention effectively compensates for phase error between successively received symbols in a detected signal.
- the possible angle differences between two consecutively received symbols over an ideal channel are ⁇ 45° and ⁇ 135°. While actual results deviate from ideal conditions due to various factors, there is a statistical concentration of differential phase angle values in the vicinity of ⁇ 45° or ⁇ 135° even in the presence of severe channel-induced distortion.
- optimum sampling timing can be obtained by determining which of the four sets of differential phase angles stored in the RAM 144 has values closest to these expected angles.
- sampling timing adjustment circuit 121 periodically processes a plurality of successively received symbols to determine optimum sampling timing based upon the phase difference between successive samples in each set of four samples.
- sampling timing adjustment is accomplished by monitoring for and eliminating phase shift due to signal distortion observed in a received signal.
- each set of samples in a signal burst (or any desired number of symbols) is separately processed so that the differential angle between successively received symbols is determined by multiplying symbol by the complex conjugate of the other.
- the resultant vector represents the differential phase angle between the symbols and thus represents the phase shift between the successive symbols due to the ⁇ /4-shifted DQPSK (i.e., a ⁇ 45° or ⁇ 135° phase shift).
- the resultant values are accumulated for each sampling instant (i.e., an entire burst or any portion thereof) and the sets of differential phase angles for each sampling timing are compared to determine as the optimum sampling timing the set having the greatest tightness of clusters about the expected angles.
- the resultant vector has an angle representing the phase shift between the successive symbols due to the ⁇ /4-shifted DQPSK (i.e., a ⁇ 45° or ⁇ 135° phase shift).
- the resultant values may optionally be rotated so that the in-phase (I) and quadrature phase (Q) components are positive and the angle of the vector is in the first quadrant.
- the angle of the rotated values in an ideal channel, should be 45°. As will be appreciated by those in the art, any angle may be used for comparison purposes.
- Deviation from 45° (or another predetermined angle), in the case of rotation, or deviation from the expected angles of ⁇ 45° or ⁇ 135°, where rotation of differential phase angles is not performed, is indicative of phase shift due to channel-induced distortion and other effects.
- the sampling timing can be optimized based upon the occurrence of phase shifts occurring in an information burst due to the time-varying channel.
- the precise number of samples per symbol that are taken by complex sampler 106 which is four in the presently described embodiment, may be set to any desired value.
- the accuracy of the sampling timing adjustment performed by the present invention depends upon the number of samples. In particularly noisy environments, therefore, the number of samples may be increased. In some embodiments, the number of samples per symbol can be a variable based upon the level of detected noise.
- the sampling timing adjustment circuit 121 operates without use of known data such as a pilot signal, training sequence or sync word. However, the present invention may also be practiced using such known data. In the preferred embodiment, however, optimum sampling timing is determined without advance knowledge of a symbol sequence, and sampling timing is determined based on the analysis of successive symbols in a received signal. In some preferred embodiments, the sampling timing adjustment circuit 121 can be used to obtain an initial sampling timing, which can be periodically updated to adjust the sampling timing due to channel effects. Alternatively, frame/slot synchronization and initial optimum sampling timing determination can be performed by other means, with or without the use of a known data sequence, and the sampling timing adjustment method and apparatus of the present invention can be applied at desired intervals to adjust the sampling timing to compensate for signal distortion.
- the deviation of the values of z(i) from the ideal phase angles noted above is determined by the optimum sampling timing set determining circuit 146 .
- each symbol there are four samples per symbol so that there are 4 streams of values of z(i), i.e., z0, z1, z2 and z3, for each symbol.
- Each z(i) value has N elements, wherein the value of N equals the number of symbols used to make a sampling timing adjustment.
- the accumulated values of z(i) for each set of samples is used to determine the Bet of samples having the smallest deviation from ideal results.
- Various methods may be used to determine the optimal set of signal samples, and all such methods are within the scope of the present invention.
- the set of samples having the smallest deviation from ideal values is determined by rotating the differential angles so that the angle thereof falls in the first quadrant (which, under ideal conditions, should be 45°). This may be accomplished by rotating the value of z(i) by either ⁇ 90° or 180° or by adding or subtracting enough multiples of 45° until the angle is closest to 45°. Then, a statistical process, such as variance, is applied to determine the tightness of each set of samples to 45°.
- the sine or cosine of each of the differential phase angles from each stream of z(i) may be compared to the sine or cosine of 45° and the optimum set of samples selected as the set having the smallest deviation therefrom. This method, however, suffers from ambiguity since two possible phase angle candidates exist for most sine or cosine values.
- Another method of determining the concentration of phase angles around ideal results is to correlate the differential phase angles with a set of predetermined angles.
- the number of predetermined angles in the set may be increased to produce a more accurate result.
- a statistical method could be applied to determine the set of, samples representing the optimum sampling timing.
- the set of samples reflecting the optimum sampling timing is output to the frequency offset estimation and compensation circuit 122 .
- the frequency offset estimator circuit 122 is the same as that disclosed in applicants' co-pending application Ser. No. 09/353009. As described in applicants' co-pending application, frequency offset estimation is performed by determining the differential phase angle between selected pairs of successive symbols. The resultant values are rotated so that the in-phase (I) and quadrature phase (Q) components are positive and the angle of the vector is in the first quadrant. As a result of rotation, the angle of the rotated vectors, in an ideal channel, should be 45°. A deviation from 45° is indicative of “phase rotation” due to frequency offset between the local reference and the carrier. Once determined, the phase rotation value can be used to adjust the local reference or the symbol sequence and thereby eliminate the frequency offset.
- the frequency offset estimator circuit 122 operates in a preferred embodiment without use of known data such as a pilot signal, training sequence or sync word. However, the present invention may also be practiced using such known data. In the preferred embodiment, frequency offset is estimated without a priori knowledge of a symbol sequence, and frequency offset is estimated and eliminated based on the analysis of successive symbols in a received signal.
- the frequency offset estimator circuit 122 includes an N-symbol delay circuit 124 which receives the symbols of the selected set of samples from the sampling timing adjustment circuit 121 and outputs a symbol delayed by N symbol periods, wherein N is an integer representing a respective received symbol, In the presently described embodiment, N is equal to 1. Accordingly, the N-symbol delay circuit 124 outputs a symbol delayed by one symbol period. Assuming that a current symbol is represented by r(i+1), wherein i is an integer representing a symbol period, the output of the N-symbol delay circuit 124 in the preferred embodiment is r(i).
- the output of the N-symbol delay circuit 124 is supplied to a complex conjugation circuit 126 , which outputs the complex conjugate of r(i) (i.e., r*(i)).
- a complex multiplier 128 receives an output of the analog-to-digital converter 106 and an output of the complex conjugation circuit 128 and multiplies the value of a current sample r(i+1) with the value of the complex conjugate of the immediately preceding sample r*(i). By this multiplication, a vector having an angle representing the differential phase angle between the successive symbol samples r(i) and r(i+1) is produced.
- the output z′ (i) of the complex multiplier 128 for the ith symbol is represented by r(i)*r(i+1).
- the phase angle of all of the values of z′ (i) will be one of the four values ⁇ 45° or ⁇ 135° in an ideal channel.
- phase angle of successive values of z′ (i) are compared to obtain a value of constant deviation from ideal conditions.
- a phase rotation circuit 130 modifies the values of z′ (i) so that they each have positive I and Q components. This is done by rotating the value of z′ (i) by either ⁇ 90° or 180°. Since the value of z′ (i) will have an angle of ⁇ 45° or ⁇ 135° under ideal conditions, the value of z′ (i) will be in the vicinity of these angles in a distorted channel so that if z′ (i) has a value in the vicinity of ⁇ 45°, rotation of z′ (i) by +90° will result in a complex vector having an angle in the vicinity of 45°.
- phase rotation results in a constant phase rotation at a frequency of 2 ⁇ , where ⁇ is the differential phase error due to frequency offset. Detection of this phase rotation facilitates the elimination of frequency offset.
- the resultant constellation of expected phase values at the output of the phase rotation circuit 130 includes vectors which have expected differential phase values of ⁇ 45° and ⁇ 135° in the phase plane, but are offset by an angle ⁇ T, which represents the phase increment due to frequency offset in one symbol period (T).
- ⁇ T represents the phase increment due to frequency offset in one symbol period (T).
- a frequency offset calculating circuit 132 receives the output of the phase rotation circuit 130 .
- a desired number of z(i) values are accumulated in, the frequency offset calculating circuit 132 and are used to detect a constant deviation from 45° (i.e., ⁇ T).
- the number of values of z(i) necessary to determine a constant deviation in ideal phase value will vary depending upon the application and may be varied depending upon the magnitude of deviation from 45° initially detected in the output of the phase rotation circuit 130 .
- phase error due to frequency offset will generally be expressed as a constant deviation from the ideal angle of 45°, while other distortion phenomenon will vary in a more random manner.
- the accumulation of values of z(i) may be stopped and the average deviation may be used to determine frequency offset based upon the average phase deviation, i.e., based on the average amount that the z(i) values deviate from 45°.
- Preferred embodiments of frequency offset determination include the averaging of several z(i) values to determine an average or constant phase deviation, and correlation of the z(i) values using a bank of unit vectors with angles in the vicinity of 45°, e.g., 45°+d, 45°+2d, 45°+3d, . . . , wherein d is an incremental angle and is set based upon the desired resolution of the system.
- the unit vector having the best correlation to the calculated value of 45°+ ⁇ T is selected.
- the frequency offset calculating circuit 132 After the phase deviation is determined, the frequency offset calculating circuit 132 outputs a value expressed as exp( ⁇ j2 ⁇ T), wherein ⁇ represents the phase deviation. This value is supplied to a complex multiplier 134 and is multiplied by the output of the analog-to-digital converter 106 to effectively adjust the frequency of the local oscillator and remove the frequency offset.
- the differential phase circuit 136 of the sampling timing adjustment circuit 121 may also serve as the circuitry used in the frequency offset estimating circuit 122 for determining the differential phase between successive symbols, including the N-symbol delay circuit 123 , the complex conjugation circuit 126 and the complex multiplier 128 .
- the circuitry and processing of the present invention may thus be shared to a great extent.
- the differential phase values of the set of samples representing the optimum sampling timing as determined by the sampling timing adjustment circuit 121 may be used by the frequency offset estimation circuit 122 to thereby avoid the need for the N-symbol delay circuit 124 , the complex conjugation circuit 126 and the complex multiplier 128 in the frequency offset estimation circuit 122 . If phase rotation is used in the sampling timing adjustment circuit, the phase rotation circuit 130 would also be included in the sampling timing adjustment circuit and would therefore be unnecessary in the frequency offset estimation circuit 122 .
- the frequency offset estimating process described above is performed by determining phase rotation in a detected signal.
- frequency offset compensation can be performed based solely upon the differential phase angle values stored in the RAM 144 , or, based on calculations performed by the optimum sampling timing set determining circuit 146 .
- the same values as calculated by the frequency offset estimating circuit 122 can be used by the sampling timing adjustment circuit 121 to determine an optimum sampling timing.
- the frequency offset estimating circuit 122 can estimate and compensate for frequency offset by merely determining a constant deviation from ideal results, as described above.
- the frequency offset estimation circuit 121 merely comprises frequency offset calculating circuit 132 which, as described above, determines a phase offset from the differential phase values z(i) and outputs a frequency offset correction value.
- a certain fixed or dynamically calculated threshold may be selected for use in the above-described process.
- One such threshold may be the average received signal power or a multiple or fraction thereof over a specified period of time, such as a frame period.
- FIG. 4 is a flowchart illustrating the method of sampling timing adjustment according to the present invention.
- the complex conjugate of a sample r*(i) above the threshold value is determined (step 402 ) and is multiplied by the value of r(i+1) (step 403 ), assuming that the value r(i+1) is also above the threshold level.
- a plurality of the resulting values of z(i) are accumulated (step 404 ). By correlating the values of vectors z(i) to determine a minimum offset from ideal conditions, an optimum sampling timing is determined (step 405 ).
- a plurality of sets of samples of a received symbols is processed as follows. Received pairs of successive symbols above a predetermined threshold value in a communication system are multiplied to produce a vector having an angle representing the differential phase angle therebetween. The resultant values are optionally rotated so that the in-phase and quadrature phase components are positive, A constant deviation from ideal phase angle values is observed, and the sampling timing corresponding to the set of samples exhibiting the smallest deviation from ideal results is selected as the optimum sampling timing.
- Sampling timing adjustment and frequency offset estimation and compensation can be performed in many different ways. For instance, a one-time sampling timing adjustment and frequency offset determination can be made for each call set-up and handoff, in which case the sampling timing and frequency offset values obtained remain unchanged most of the time.
- the sampling timing adjustment circuit 121 and the frequency offset estimating circuit 122 begin accumulating z(i) values to update their results.
- sampling timing adjustment and frequency offset compensation can be performed to compensate for crystal oscillator inaccuracies or drift, such calculations being necessary on a periodic basis.
- offset can be compensated for more frequently than in environments in which offset amount is small.
- a predetermined symbol sequence can be stored in memory, and when the same sequence is detected in an incoming signal, the symbol multiplication values z(i) for the received sequence can be compared to similar values based.
- a sync word, pilot sequence, training sequence, or the like may be used as the predetermined sequence.
Abstract
Description
Claims (13)
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US09/371,529 US6567480B1 (en) | 1999-08-10 | 1999-08-10 | Method and apparatus for sampling timing adjustment and frequency offset compensation |
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